1. Field of the Invention
The subject invention relates to digital circuitry and, more particularly, to a low-cost 16-bit synchronous counter implemented in GaAs technology that achieves speeds in excess of 600 Mhz over the military temperature range of -55 to 125.degree. C.
2. Description of Related Art
Conventional synchronous counters are comprised of a series of registers configured through the interconnection of AND gates, EX-OR gates, and D-type flip-flops. In operation, each of the registers contains a binary digit, which together represent the number of pulses counted. The speed at which the counters can perform is determined, in large part, by the speed at which the components of the counters can react. Typical delays encountered include gate delays (the time it takes an electronic gate to react), flip-flop delays (the time it takes a flip-flop to change to the desired output), interconnect delays (the time it takes the signal to travel between components), and setup delays (the time an input signal to a flip-flop must be held while waiting for the clock pulse). Each of the delays encountered in producing a carry signal reduces the speed performance of the counter.
In general, a carry signal is generated when the count contained in a particular group of registers has reached the limit of that group of registers (e.g., each digit is a one so the registers are "full"). The carry signal generated indicates to the next group of registers that one count is to be added. At the next clock pulse, one count is added to the contents of the next group of registers and the first group of registers is reset to zero.
Where the carry signal is not generated until the group of registers is full, the speed of the counter is constrained by the time it takes for the counter to execute the carry and reset the appropriate registers before it can resume counting. Therefore, to the extent that the carry signal can be generated before it is needed, and be applied at precisely the time required, this delay is avoided and the speed performance of the counter can be improved. Look-ahead carry circuitry for reducing delays and, hence, increasing counter speed is also known in the art, as disclosed in U.S. Pat. No. 5,062,126 assigned to the present assignee.
Gallium arsenide is a known device technology for producing high-speed operation. However, application of conventional counter design to gallium arsenide technology, even employing look-ahead carry, cannot achieve counting speeds which approach 600 MHz over a range of -55 to 125.degree. C. Such counter speeds appear desirable to advance the state of the art in various apparatus such as laser range finder systems requiring better than one meter resolution.